M68ICS08GR datasheet(110/408 Pages) FREESCALE

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Clock Generator Module (CGMC)

Technical Data

MC68HC908GR8 — Rev 4.0

110

Clock Generator Module (CGMC)

MOTOROLA

7.4.8 Base Clock Selector Circuit

This circuit is used to select either the crystal clock, CGMXCLK, or the

VCO clock, CGMVCLK, as the source of the base clock, CGMOUT. The

two input clocks go through a transition control circuit that waits up to

three CGMXCLK cycles and three CGMVCLK cycles to change from

one clock source to the other. During this time, CGMOUT is held in

stasis. The output of the transition control circuit is then divided by two

to correct the duty cycle. Therefore, the bus clock frequency, which is

one-half of the base clock frequency, is one-fourth the frequency of the

selected clock (CGMXCLK or CGMVCLK).

The BCS bit in the PLL control register (PCTL) selects which clock drives

CGMOUT. The VCO clock cannot be selected as the base clock source

if the PLL is not turned on. The PLL cannot be turned off if the VCO clock

is selected. The PLL cannot be turned on or off simultaneously with the

selection or deselection of the VCO clock. The VCO clock also cannot

be selected as the base clock source if the factor L is programmed to a

0. This value would set up a condition inconsistent with the operation of

the PLL, so that the PLL would be disabled and the crystal clock would

be forced as the source of the base clock.

7.4.9 CGMC External Connections

In its typical configuration, the CGMC requires up to nine external

components. Five of these are for the crystal oscillator and two or four

are for the PLL.

The crystal oscillator is normally connected in a Pierce oscillator

configuration, as shown in Figure 7-2. Figure 7-2 shows only the logical

representation of the internal components and may not represent actual

circuitry. The oscillator configuration uses five components:

•

Crystal, X1

•

Fixed capacitor, C1

•

Tuning capacitor, C2 (can also be a fixed capacitor)

•

Feedback resistor, RB

•

Series resistor, RS

Freescale Semiconductor, Inc.

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